Overdrive Operation and Basic Trouble-shooting BN4-BJ8
With some assistance from our schematic circuit diagram, we can examine how the
overdrive electrical circuit looks under various scenarios. This is also a good procedure
to use for checking operation of the electrical system in the event the overdrive isnĎt
As shown in Diagram B, with the ignition on, the engine not running, and the gear lever
in third or fourth ,when the dash switch is moved from NORMAL to OVERDRIVE, you
should hear a distinct click from the overdrive as the solenoid is energized and the
plunger is pulled up. If you donít hear the click, then some component of the electrical
system is faulty, and youíll need to check each component in turn as discussed below.
If you do hear the click, but the overdrive doesnít work when the car is driven, then the
problem is in the overdrive itself, which will have to be checked by a qualified specialist.
Diagram C illustrates what happens when the dashboard switch is turned off while the
throttle pedal is less than one-quarter pressed. The throttle switch points are closed,
keeping the solenoid coil energized, maintaining contact between the points on C1 and
C2 so that current continues to flow to the solenoid. The overdrive will stay engaged
until such time as you depress the throttle pedal enough to open the throttle switch
points, thus de-energizing the circuit - as shown in Diagram D. You can use this feature
to anticipate down-shifting out of overdrive. For example, if you are driving on fairly
level ground, you can flip the dash switch to NORMAL and still maintain your speed
using only a very light throttle.
The overdrive will remain engaged. When you are ready to shift out of overdrive,
merely depress the throttle about one-quarter and the transmission will immediately
down-shift out of overdrive. This procedure can be handy when exiting a freeway, as it
only takes a light ďblipĒ of the throttle as you enter the off ramp to drop out of overdrive.
To check the throttle switch adjustment, with the engine off and the ignition turned on,
flip the overdrive switch on. Then, with your foot off the throttle, flip the switch to
NORMAL. Now flip the switch back on again. You should not hear the overdrive
solenoid click, as it should not have been de-energized when the switch was flipped off.
Next, repeat this above sequence, except this time depresses the throttle about one-
quarter before flipping the switch on for the second time. You should now hear the
solenoid energize, as depressing the throttle should have opened the throttle switch
A more accurate and direct way to check the throttle switch setting is to disconnect the
two wires and put an ohmmeter across the terminals. The reading should go from zero
ohms (throttle less than one-quarter depressed switch points closed) to infinite when
the points open.
To adjust the throttle switch, loosen the clamp bolt on the lever arm. The throttle switch
has an internal spring which helps in setting it. With the shaft free to be positioned by
this spring, the cam should be oriented so that the points are closed.
You can check this by using a screwdriver to rotate the shaft by its slot. Turning an
equal amount clockwise and then counterclockwise you should find a position in each
direction where the resistance becomes infinite - the points are now open. Position the
shaft about half way between these extremes, tighten the arm clamp, and use this as a
starting point for adjustment. Observe the throttle position when the resistance changes
from zero, and make adjustments to the shaft position until this occurs at about one-
Diagram E illustrates what happens if the transmission is shifted down into second or
first, as for example if you have to slow down on the highway in a traffic jam. In this
case, there is still power flowing through relay coil and consequently through the C1
and C2 points but the overdrive solenoid circuit is open at the gear shift switch,
preventing the power from getting to the solenoid coil. As soon as you shift back into
third, the overdrive will re-engage.
Further Electrical Trouble-shooting
The gear shift switch operation can be checked as follows. Disconnect the ground
wire from terminal W2 on the overdrive relay and connect a jumper wire (with alligator
clips at both ends) from terminal A3 on the fuse block to C2 on the overdrive relay.
Switch on the ignition (but do not start the engine) and move the gear shift lever back
and forth from the three-four slot to the first-second slot. Each time the lever is moved
over to the three-four position, you should hear the solenoid click as the plunger is
pulled up. If you donít hear that click, then the throttle switch is faulty. Before continuing
with the following additional tests remove the jumper wire and reconnect the ground
wire to W2.If the gear lever switch is working properly, to check the overdrive relay,
turn on the ignition and shift the transmission into third or fourth gear. In the engine
compartment, attach one end of a short jumper wire from terminal W1 of the relay and
touch the other end to fuse post A3. If you donít hear the relay respond, the internal coil
is bad. You can also sense operation of the relay by feeling the cover for vibration. If
you verify the relay is working, then go listen for the overdrive solenoid to click while
someone else touches the jumper wire to the terminals. If the solenoid doesnít operate,
then the relay points could be dirty or damaged and not making good electrical contact.
Once you have verified that all the other components are working properly, you can
check the dash toggle switch. With the engine off, ignition turned on, gear lever in one
of the top two gears, and throttle pedal released, flip the toggle to OVERDRIVE. You
should hear the solenoid click. Flip the switch back to NORMAL and turn the ignition
switch off (to de-energize the solenoid even if the throttle switch is not set correctly).
Re-test by turning the ignition switch on followed by the toggle switch.
If the overdrive should stop operating while youíre out on the road and you can
determine that itís an electrical problem rather than a problem within the overdrive itself,
it may be tempting to bypass the offending component(s).
However, since these components are designed to protect the overdrive, this may not
be a good idea, since it can be considerably more expensive to replace the overdrive
should that fail as a consequence of one of the override switches not working.
Your engine can quite effectively handle highway speeds even without overdrive.
Though the higher rpm may be a little wearing on driver and passenger, the car will get
you to a place where you can trouble-shoot the system properly and properly replace a
failed electrical component.